1. Field of the Invention
The present invention relates to novel molding compositions comprising a polymer matrix and an inorganic reinforcing filler therefor, in which the mineral filler is bonded to the polymer, if appropriate by means of a coupling agent. The present invention more especially relates to molding compositions based on reinforced polymers of a thermoplastic, thermosetting or elastomeric nature, in which the mineral filler exhibits an improved bonding capacity. The present invention also relates to processes for the preparation of such compositions, means adapted for carrying out such processes, and the shaped articles obtained via the molding thereof.
2. Description of the Prior Art
It is well known to this art that polymers can contain non-polymeric materials as fillers, in order to improve certain of their properties. Thus, various mineral or inorganic fillers can be used, in particular in order to improve the mechanical properties of the polymer, for example, its expansion coefficient and/or its abrasion resistance and/or its modulus and/or its tensile strength and/or its flexural strength and/or its resilience. The key to the development of such compositions containing fillers resides in establishing an effective bond between the filler and the polymer. Without this bond, the addition of the filler may even impair certain mechanical properties of the polymer.
To establish this effective bond, the nature of the filler will be changed, but it is envisaged, if necessary, to additionally employ an effective coupling agent, namely, a compound which will be able to react both with the polymer and with the inorganic filler. It must possess at least one group capable of reacting with the polymer and at least one other group capable of reacting with the functional groups present on the surface of the filler (usually OH groups). Certain organosilicon compounds have been used as coupling agents. Suitable compounds of this type are described, for example, in French Patent No. 1,399,049; as groups capable of bonding to the filler, same typically comprise alkoxysilane groups -Si-OR, in which R represents a lower alkyl radical.
In general, not all mineral fillers are equally effective as reinforcing fillers. A necessary condition for this efficacy is the existence, on the surface of the filler, of an adequate density of functional groups which will be able to react with the polymer, and if appropriate, the coupling agent. Numerous mineral fillers have been proposed, among which the most representative consist of the following species: calcium carbonate, titanium oxide, micronized silica, an aluminosilicate such as calcined kaolin, a magnesosilicate such as talc, wollastonite and calcium sulfate.
Experiments conducted by the assignee hereof employing nylon 6,6 as the polymer matrix have resulted in the observation that calcined kaolin and talc would appear to be the fillers which make it possible to obtain the best compromise in terms of mechanical properties; it should be added that talc is preferred because such a filler results in less shrinkage of the shaped articles molded therefrom.